dmaengine: imx-sdma: add missing structure description

Some member description or colons missing cause build warning with
'W=1' as below:

drivers/dma/imx-sdma.c:326: warning: Function parameter or member 'vd' not described in 'sdma_desc'
drivers/dma/imx-sdma.c:326: warning: Function parameter or member 'num_bd' not described in 'sdma_desc'
drivers/dma/imx-sdma.c:326: warning: Function parameter or member 'bd_phys' not described in 'sdma_desc'
drivers/dma/imx-sdma.c:326: warning: Function parameter or member 'buf_tail' not described in 'sdma_desc'
drivers/dma/imx-sdma.c:326: warning: Function parameter or member 'buf_ptail' not described in 'sdma_desc'
drivers/dma/imx-sdma.c:326: warning: Function parameter or member 'period_len' not described in 'sdma_desc'
drivers/dma/imx-sdma.c:326: warning: Function parameter or member 'chn_real_count' not described in 'sdma_desc'
drivers/dma/imx-sdma.c:326: warning: Function parameter or member 'chn_count' not described in 'sdma_desc

Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Reported-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>